(1) Field of the Invention
The present invention relates to a cooling water pump device for pumping cooling water toward an engine of an outboard motor that includes a hollow driveshaft housing under the engine and a driving shaft vertically mounted in the driveshaft housing for transmitting the drive force of the crankshaft of the engine to a screw.
(2) Description of the Prior Art
The outboard motor engine is cooled by taking in seawater or river water through, for example, a water filter in the lower case (or gear case) and forwarding the intake seawater or river water to the water jacket of the engine as cooling water.
In general, an outboard motor is equipped with a cooling water pump device for sending (pumping up) cooling water for engine cooling.
Specifically, an outboard motor is provided under its engine with a driveshaft housing that incorporates a driving shaft mounted vertically for transmitting the drive force of the crankshaft of the engine to a screw. The outboard motor has a cooling water pump device (water pump) which by using an impeller made of elastic material, accommodated eccentrically in the pump case, at a position partway through the length of the driveshaft, pushes cooling water forwards to the engine by rotation of the impeller inside the pump case as the driveshaft is driven (see Japanese Patent Application Laid-open Hei 5 No. 306687 and Japanese Utility Model Application Laid-open Hei 2 No. 126992).
As stated above, the cooling water pump device of an outboard motor employs a so-called water cooling type which draws cooling water and pushes it forward to the engine side so as to cool the engine with the thus pumped cooling water. In general, almost all models of outboard motors, from compact models (low horsepower models) as low as 2 horsepower (2 hp) to large-scale models (high horsepower models) as high as 250 horsepower employ water cooled engines that use a cooling water pump device.
The material of the pump case used for cooling water pump devices can be classified roughly into stainless steel and resin. As specific configurations, FIG. 15 shows a cooling water pump device with a pump case “b” formed of stainless steel and FIG. 16 shows a cooling water pump device with a pump case “b” of resin.
Either of these cooling water pump devices shown in FIGS. 15 and 16 is assembled around a driveshaft “a” of the outboard motor, and an impeller “c” of elastic material is arranged eccentrically inside the pump case “b” and is fixed to driveshaft “a” by a key “d” with respect to the direction of rotation.
As the impeller “c” is rotationally driven as driveshaft “a” turns, water for cooling is drawn in from the outside of the outboard motor through an inlet port (not shown) of a lower case “e” (also called a gear case: accommodating gears and a screw shaft) located at the bottom of driveshaft “a” and pumped toward the engine. Concerning each pump case “b”, in order to secure watertightness at the contact face with lower case “e”, the pump case “b” is mounted by interposing an under-panel “f” and a gasket “g” between the underside of pump case “b” and the top side of lower case “e”.
In the case of a cooling water pump device of the type shown in FIG. 15, using a stainless pump case “b”, it can present a high enough strength against sliding of impeller “c”. On the other hand, in the case of a cooling water pump device of the type shown in FIG. 16, using a resin pump case “b”, a sleeve “h” made of metal such as stainless steel, is fitted to the pump case “b” side which impeller “c” comes into sliding contact with so as to prevent abrasion of pump case “b” due to rotation of impeller “c”. Further, an O-ring “i” is held between the abutment faces of resin pump case “b” and under-panel “f” and fixed by bolts.
In contrast with this, in the stainless pump case of the type shown in FIG. 15, the fitting surface of the pump case to the under-panel “f” is flattened so that no O-ring is used at the interface.
The advantage of using a stainless pump case for the cooling water pump device of an outboard motor is that when the engine is started in the dry for maintenance of the outboard motor, no deficiency such as the onset of case fusing will occur if the impeller “c” rotates and generates heat at its sliding surface with the pump case due to lack of cooling water. Therefore, it is possible to use the outboard motor in an ordinary manner after checkup with the engine started. Further, as will be described later, no metal sleeve is used as used for resin pump cases, hence there is no possibility of salt building up between the pump case and the metal sleeve and producing cracks that might cause the sleeve to move toward the case.
Because of these advantages, conventional pump cases, in general, have been made of stainless steel.
However, a pump case made of stainless steel suffers from various drawbacks: it is heavier than the that made of resin, causing a hindrance to lightening of the engine; and it is usually produced using the lost wax process, which is poor in mass productivity and needs high material cost and processing cost, resulting increase in cost.
For the above reasons, recently there has been a trend toward using resin pump cases. There are various advantages of using a resin pump case: it can be configured of a reduced number of parts because its parts can be integrally formed within limits and hence it is preferable for mass production; the weight of the pump case is lighter than that made of stainless steel or other metal, so that the pump, hence the outboard motor can be readily lightened; and the cost is low because the materials are inexpensive and the processing cost is low.
However, the resin pump case tends to deform due to heat during the operation in the dry. Further, for the case where outboard motors are used in seawater, saltwater may enter the interface between the pump case and the metal sleeve, forming salt buildup which may cause cracks in the case and deformation of the metal sleeve.
As a measure to prevent infiltration of salt water into the interface between the pump case and metal sleeve, a sealant for water protection may be applied between the metal sleeve and the pump case.
However, the applied amount of sealant may vary depending on the worker. Use of an automatic sealant coater to deal with this results in cost increase. And also, the sealant effectiveness will become lower due to heat and aging. Further, when the metal sleeve is to be replaced, adhesion of sealant is hard to peel off, increasing the workload. Moreover, when a new part is to be assembled, sealant is to be applied at a site in the local dealer, resulting in increase the number of steps and yet lack of reliability.